Project Summary/Abstract The dendritic trees of neurons in the brain's cortex conduct elemental brain functions such as information processing and learning. Research in brain slices has provided a wealth of information about the dendritic mechanisms that could be operating in behaving animals to integrate, and alter the strength of, synaptic inputs from presynaptic sources. Key among these mechanisms are back-propagating action potentials and nonlinear integration of synaptic inputs leading to dendritic spiking, which confer to the dendritic tree a host of local and global plasticity and signaling possibilities. Numerous alluring models of information processing and learning rules have arisen as a result of these in vitro findings, and supported by this R01, we have begun to determine which of these mechanisms are at work in awake, behaving animals. The present proposal leverages recent technical advances used and developed by the PI that enable functional imaging of calcium transients with single dendritic branch resolution and of glutamate input with synaptic scale resolution in the hippocampus of head-restrained mice performing spatial behaviors in a virtual-reality interface. Using these methods, the research proposed in this grant application will allow us to bridge two disconnected areas of neuroscience research: research characterizing the firing patterns and changes in firing patterns of hippocampal neurons during behavior, and research in reduced preparations investigating the mechanisms underlying firing and synaptic plasticity. Specifically, we aim to determine the behavioral relevance and synaptic basis of hippocampal dSpikes. This will allow testing of models of plasticity which have been developed based on in vitro data, across a wide range of parameter space, to finally establish which learning mechanisms are behaviorally relevant.